Method for converting uo3 or u3o8 into hydrated uo4

ABSTRACT

A method for converting UO 3  and/or U 3 O 8  into hydrated UO 4  of formula UO 4 .nH 2 O wherein n is 2 or 4, comprising the following successive steps:
         a) preparing an aqueous suspension of a UO 3  powder and/or a U 3 O 8  powder;   b) adding hydrogen peroxide H 2 O 2  to the aqueous suspension of a UO 3  and/or U 3 O 8  powder, converting the UO 3  and/or U 3 O 8  into hydrated UO 4  and precipitating, crystallizing the hydrated UO 4  in the suspension;   g) recovering the precipitate, crystals of UO 4  hydrate;   h) optionally, washing the recovered UO 4  hydrate precipitate, crystal(s);   i) optionally, repeating step d);   j) optionally, drying the precipitate, the crystals;
 
wherein the addition of H 2 O 2  to the aqueous suspension is carried out so that the suspension contains a stoichiometric excess of H 2 O 2  relatively to the stoichiometry of the reaction from UO 3 :
       

       UO 3   +n H 2 O+H 2 O 2 →UO 4   .n H 2 O+H 2 O  (1)
 
     or of the reaction from U 3 O 8    
       UO 2.67 +1.33H 2 O 2   +n H 2 O→UO 4   .n H 2 O+H 2 O  (2)
         and the pH of the suspension is maintained in steps a) and b) at a value comprised between 2 and 3.

TECHNICAL FIELD

The invention relates to a method for converting UO₃ or U₃O₈ intohydrated UO₄ (UO₄ hydrate).

STATE OF THE PRIOR ART

The treatment of uranium ores has the purpose of extracting uranium fromthe ores, of purifying it and of combining it so as to obtain a productcalled a concentrate, or uranate or further a

yellow cake

rich in uranium, for example comprising more than 70% by weight ofuranium.

Uranium ores are first of all crushed, and then milled, and they arethen subject to an operation for putting the uranium into solution bymeans of a base or an acid, such as for example sodium carbonate orsulfuric acid, called etching or leaching.

After purification and concentration of the solutions from the leachingof the ore, the uranium is recovered in the form of uraniferous,uraniated, generally acid, liquors, solutions, in a sulfates medium forexample.

These solutions may also be in a chloride, ammonia, nitrate or carbonatemedium according to the preliminary purification-concentration step.

The uranium concentrate or

yellow cake

is obtained from these uraniferous liquors, solutions, by precipitationwith precipitation reagents such as soda, magnesia, ammonia, ammoniumuranyl tricarbonate, and hydrogen peroxide H₂O₂, by filtration anddrying.

According to the precipitation reagent used, the uranium concentrate or

yellow cake

will thus be respectively based on sodium uranate, magnesium uranate,ammonium diuranate, ammonium uranyl tricarbonate, or uranium peroxide.

The thereby prepared uranium concentrate or

yellow cake

is then transformed, notably into UF₄ and then into UF₆.

Uraniferous concentrates such as

yellow cake

, the preparation of which was described above, but also of otheruraniferous concentrates containing uraniferous uranium trioxide oruranium octa-oxide are not able to be directly converted notably intoUF₆.

Indeed, they contain too many impurities for the subsequent isotopicseparation step (also designated as enrichment) relatively to the ASTMenrichment standards on the one hand, the presence of certain compoundsmay be redhibitory for the fluorination method on the other hand.

Moreover, certain oxides have insufficient reactivity for thereduction/hydrofluorination step leading to UF₄.

In order to find a remedy to all these problems, the concentrates arepurified before converting them.

A purification method is thus known in which the

yellow cake

is, first of all, dissolved in nitric acid and the solution is then sentinto a counter current liquid-liquid extraction apparatus in which theuranyl nitrate of the solution is extracted by using a mixture of TBPand of kerosine.

This method is complex and uses nitrates and volatile organic compoundswhich have to be handled.

A treatment of the used solvent and of the nitrate effluents has notablyto be carried out.

The method for precipitating uranium with H₂O₂ is known to bedecontaminating towards many impurities.

However, all the known methods for precipitating uranium with H₂O₂ applypreliminary dissolution of the uranium, before reprecipitation ofUO₄.4H₂O by addition of oxygenated water.

However, this technique has the two following major drawbacks:

-   -   the addition of an acid and therefore of the associated anions        such as sulfates, chlorides, nitrates, etc. during the        dissolution step. These anions are impurities which prove to be        bothersome in the subsequent conversion steps,    -   the addition of a base and therefore of the associated cations        such as sodium, potassium, ammonium, cations during        precipitation of hydrated UO₄ in order to maintain the pH        constant. These cations are impurities also very bothersome for        the formation of UF₄ (Na, K . . . ), or else generate gaseous        effluents.

Certain added impurities may partly follow the uranium, in spite ofrepeated washings, and are bothersome for the UF₄ conversion method.

In particular, sodium and potassium form eutectics while the sulfatesrelease corrosive H₂S.

Among the documents which describe precipitation from concentrates withH₂O₂, for purposes of purification, mention may be made of documentWO-A1-2009/013759, which describes a method for refining

yellow cake

in order to prepare uranium of nuclear quality in which precipitation isachieved in a single step in order to simultaneously remove heavymetals, boron and other rare earth metals. In this method, one beginswith dissolving the

yellow cake

in nitric acid with moderate stirring in order to produce a solution ofuranyl nitrate, and hydrogen peroxide is added at a predefinedtemperature and pH in order to selectively precipitate hydrated uraniumperoxide.

The method of this document includes a preliminary step for dissolvingthe concentrate with nitric acid with all the drawbacks of such a stepas listed above.

Document FR-A-2 438 623 relates to a method for purifying hydrateduranium(VI) peroxide wherein a uranium concentrate is digested in anacid aqueous solution, notably a solution of nitric acid, in thepresence of a fluoride-complexing agent so as to obtain an aqueoussolution of uranium, and this aqueous solution of uranium is reactedwith a peroxide in order to precipitate hydrated uranium(VI) peroxide.

Again, the method of this document includes a preliminary step fordissolving the concentrate with an acid such as nitric acid with all thedrawbacks of such a step as listed above.

Document FR-A-2 429 747 relates to a method for preparing hydrateduranium(VI) peroxide from hydrated uranium tetrafluoride, in which thehydrated uranium tetrafluoride is digested in an acid solution, notablya solution of nitric acid, in the presence of an agent for precipitatingfluorides, for precipitating fluoride ions and obtaining an aqueoussolution of uranium, the aqueous solution of uranium is filtered and thepH is adjusted and the aqueous uranium solution is reacted with aperoxide in order to precipitate the hydrated uranium(VI) peroxide.

There again, the method of this document includes a preliminary step fordissolving the concentrate with an acid, notably with nitric acid, withall the drawbacks of such a step as listed above.

Therefore considering the foregoing, there exists a need for a methodfor converting UO₃ or U₃O₈ into hydrated UO₄ which allows preparation ofan hydrated uranium peroxide which has a low impurity content, inparticular a content of impurities which is sufficiently low so thatthis hydrated uranium peroxide may be directly converted into UF₄ andthen into UF₆.

More specifically, there exists a need for such a method which allowspreparation of UO₄ hydrate which totally or for a major part meets theASTM C-787 standard relating to the purity of hydrated UO₄ forconversion into UF₆.

This method should also allow preparation of hydrated uranium peroxidehaving a high specific surface area and great reactivity with view toits conversion into UF₄.

There further exists a need for such a method which is simple, reliable,safe and which includes a limited number of steps.

There also exists a need for a method which uses non-toxic reagents, notcausing any harm to the environment and of low cost.

The goal of the present invention is to provide a method for convertingUO₃ or U₃O₈ into hydrated UO₄ which meets the whole of the needs andrequirements, as listed above.

The goal of the present invention is also to provide such a method whichdoes not have the drawbacks, defects, limitations and disadvantages ofthe methods of the prior art, such as notably illustrated by thedocuments mentioned above, and which solves the problems of the methodsof the prior art.

SUMMARY OF THE INVENTION

This goal, and further other ones are attained according to theinvention with a method for converting UO₃ and/or U₃O₈ into hydrated UO₄of formula UO₄.nH₂O wherein n is 2 or 4, comprising the followingsuccessive steps:

a) preparing an aqueous suspension of a UO₃ powder and/of a U₃O₈ powder;

b) adding hydrogen peroxide H₂O₂ to the aqueous suspension of a UO₃and/or U₃O₈ powder, converting the UO₃ and/or the U₃O₈ into hydrated UO₄and precipitating, crystallizing UO₄ hydrate in the suspension;

c) recovering the hydrated UO₄ precipitate, crystals;

d) optionally, washing the recovered hydrated UO₄ precipitate, crystals;

e) optionally, repeating step d);

f) optionally drying the precipitate, the crystals;

wherein addition of H₂O₂ to the aqueous suspension is carried out sothat the suspension contains a stoichiometric excess of H₂O₂ relativelyto the stoichiometry of the reaction from UO₃:

UO₃+H₂O₂ +nH₂O→UO₄ .nH₂O+H₂O  (1)

or of the reaction from U₃O₈:

UO_(2.67)+1.33H₂O₂ +nH₂O→UO₄ .nH₂O+H₂O  (2),

and the pH of the suspension is maintained in steps a) and b) at a valuecomprised between 2 and 3 (2 and 3 inclusive).

Advantageously, the pH of the suspension is adjusted during step a) to avalue comprised between 2 and 3 by adding an acid to the suspension.

Advantageously, said acid is selected from oxalic acid, sulfuric acidand mixtures thereof.

Advantageously, the stoichiometric excess of H₂O₂ is from more than 1 to10, preferably from 1.5 to 3, relatively to the stoichiometry of thereaction (1), and from more than 1.33 to 10 relatively to thestoichiometry of the reaction (2).

Advantageously, the hydrogen peroxide is added in the form of an aqueoussolution at a concentration from 30% to 70% by weight.

Advantageously, the aqueous suspension of UO₃ and/or U₃O₈ has a uraniumconcentration from 10 to 500 g/L (gU/L), preferably from 100 to 200 g/Lfor UO₃, and from 10 to 500 g/L, preferably from 100 to 200 g/L forexample 250 g/L for U₃O₈.

Advantageously, steps a) and b) may be carried out with stirring.

Advantageously, during step a) and/or step b), complexing anions areadded to the suspension.

Advantageously, said complexing anions are selected from sulfate anions,oxalate anions and mixtures thereof.

Advantageously, the duration of step b) is selected so that theconversion of UO₃ and/or of U₃O₈ into hydrated UO₄ is total orsubstantially total, for example of more than 99%, or even 99.9%.

In an embodiment, step b) may comprise the following successive stepsb1) and b2):

b1) adding hydrogen peroxide H₂O₂ to the aqueous suspension of a UO₃and/or U₃O₈ powder, preferably with stirring, and then stopping theaddition;

b2) ripening the suspension, preferably with stirring.

Advantageously, the duration of said step b1) may be from 1 to 8 hours,preferably from 1 to 3 hours, and the duration of step b2) may be from 1to 24 hours, preferably from 1 to 3 hours.

In another embodiment, the addition of hydrogen peroxide H₂O₂ isachieved during the whole duration of step b), i.e. step b2) is omitted.

In this embodiment, the duration of step b) is generally from 1 to 8hours, preferably from 1 to 5 hours.

Advantageously, during steps a) and/or b), the suspension is subject tothe action of ultrasonic waves.

Notably in this case, the water of the suspension may be removed byevaporation, and the precipitate, the crystals of hydrated UO₄, are thenrecovered as a dry solid, for example with humidity less than 7% bymass, generally consisting of UO₄.2H₂O, or else during step c), theprecipitate, the crystals of UO₄ hydrate are separated from thesuspension by a solid/liquid separation operation, for example afiltration or centrifugation operation, in the form of a humid solid,for example with a humidity from 30 to 80% by mass, generally consistingof UO₄.4H₂O. On the other hand, the evaporation will not generally allowremoval of the impurities.

Advantageously, said humid solid is washed at least once with a washingliquid.

Advantageously, said washing liquid is selected from among demineralizedwater; acidified aqueous solutions preferably at a pH from 2 to 3, forexample with sulfuric acid; solutions containing an agent complexing theimpurities contained in the humid solid.

Advantageously, the washing ratio defined by the ratio of the mass ofthe washing liquid to the mass of the humid solid is from 1 to 30,preferably from 1 to 10.

Advantageously, the oxide UO₃ and/or the oxide U₃O₈ appear in the formof a uraniferous concentrate called a

yellow cake

, or the oxide UO₃ and/or the oxide U₃O₈ stem from the drying, and thenfrom the calcination of a uranium concentrate based for example on UO₄hydrate, ammonium diuranate, or uranium tricarbonate obtained byprecipitation in a reactor, notably in a fluidized bed reactor, from anuraniferous solution.

The method according to the invention may be defined as a method fordirect conversion without preliminary dissolution of U₃O₈ and/or of UO₃by addition of H₂O₂ to an aqueous suspension of a U₃O₈ powder and/or ofa UO₃ powder.

The method according to the invention includes a sequence of specificsteps which has never been described in the prior art.

The method according to the invention is fundamentally distinguishedfrom the method of the prior art in that no preliminary dissolution ofU₃O₈ and/or UO₃ is carried out before achieving their conversion byadding hydrogen peroxide.

The conversion of U₃O₈ and/or UO₃ into hydrated UO₄ (UO₄ hydrate) isthus, in the method according to the invention, achieved in a dispersionand not in a solution.

The method according to the invention, which does not include anypreliminary step for dissolving the uranium, does not have all thedrawbacks due to this preliminary dissolution step. In particular, themethod according to the invention thus avoids the formation of manyimpurities which may prove to be extremely bothersome in the subsequentsteps for conversion of UO₄ hydrate, for example into UF₄.

All the comparable methods of the prior art include such a dissolutionstep, and there does not exist any indication in the prior art whichwould have lead the man skilled in the art to suppressing thisdissolution step.

The method according to the invention is further defined by the factthat the suspension contains a stoichiometric excess of H₂O₂ relativelyto the reactions (1) and (2), which gives the possibility of obtainingtotal or quasi-total conversion.

The method according to the invention is further characterized in thatthe pH of the suspension is maintained in steps a) and b) at a specificvalue comprised between 2 and 3.

The selection of this very narrow PH range gives the possibility ofavoiding risks of redissolution of UO₄ hydrate at too acid pH's,generally less than 2, on the one hand, and of avoiding the risks offormation of compounds other than UO₄ hydrate at more basic pHs,generally greater than 3, as well as the precipitation of impuritiesfollowing the uranium, on the other hand.

The method according to the invention does not have the drawbacks of themethod of the prior art and provides a solution to the problems of theprior art.

Thus, the method according to the invention allows preparation ofuranium peroxide or of uranium peroxide hydrate which has a low contentof impurities, in particular a sufficiently low impurity content so thatthis uranium peroxide or this uranium peroxide hydrate may be directlyconverted into UF₄ and then into UF₆.

The method according to the invention notably allows preparation of UO₄hydrate which totally or for a major part meets the ASTM C-787 standardrelating to the purity of UO₄ hydrate for conversion into UF₆.

The method according to the invention further allows preparation of auranium peroxide which has high reactivity for rapid conversion intoUF₄.

Indeed, the method according to the invention gives the possibility ofobtaining a UO₄ hydrate having a high specific surface area, which mayrange up to 30 m²/g.

The invention will now be described in a detailed way in the detaileddescription which follows notably in connection with preferredembodiments. This description is given as an illustration and not as alimitation, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the DRX spectrum of the purified uranium powder inthe form of UO₄.2H₂O as obtained in Example 1.

FIG. 2 illustrates the DRX spectrum of the purified uranium powder inthe form of UO₄.2H₂O as obtained in Example 2.

FIG. 3 illustrates the DRX spectrum of the purified uranium powder inthe form of UO₄.2H₂O as obtained in Example 4.

FIG. 4 is a photograph taken with a scanning electron microscope (SEM)of nanometric needles of UO₄.2H₂O with a size of 200 nm, as obtained inExample 1.

The scale indicated in FIG. 4 represents 200 nm.

FIG. 5 is a photograph taken with a scanning electron microscope ofneedles of UO₄.2H₂O with a size from 1 μm to 2 μm, as obtained inExample 2.

The scale indicated in FIG. 5 represents 200 nm.

FIG. 6 is a photograph taken with a scanning electron microscope ofagglomerates of UO₄.2H₂O with a size from 100 nm to 200 nm, as obtainedin Example 6.

The scale indicated in FIG. 6 represents 200 nm.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the first step of the method according to the invention, an aqueoussuspension of a uranium trioxide UO₃ powder and/or of a uraniumocta-oxide U₃O₈ powder is prepared.

The method according to the invention may be applied with all kinds ofuranium trioxide UO₃ and/or uranium octa-oxide U₃O₈, regardless of theirorigin and of the shape in which they appear.

These oxides may for example appear as concentrates called

yellow cake

.

These oxides may also stem from drying, and then from calcination of auranium concentrate based for example on UO₄ hydrate, ammoniumdiuranate, or uranium tricarbonate by precipitation in a reactor,notably in a fluidized bed reactor, from an uraniferous solution.

A method for preparing a UO₃ powder or a U₃O₈ powder by drying and thenby calcination of a uranium concentrate based on hydrated UO₄, onammonium diuranate or on uranium tricarbonate obtained beforehand byprecipitation in a fluidized bed, is described in documentWO-A1-2010/051855 to the description of which reference may be made.

The UO₃ or U₃O₈ powders obtained in this document because of thepreparation of the uranium concentrate in a fluidized bed, haveparticularly advantageous properties.

The oxides in the form of concentrates called

yellow cake

or the oxides stemming from drying and calcination of a uraniumconcentrate obtained preferably by precipitation in a fluidized bedgenerally appear as powders and may be directly used in the methodaccording to the invention, and be suspended in water.

However, it may be advantageous to carry out preliminary milling of theoxide powders in order to obtain a particularly fine grain size, forexample of the order of one micrometer.

The suspended powders generally contain impurities and the methodaccording to the invention notably has the purpose of reducing thecontent of these impurities in the obtained hydrated uranium peroxide(uranium peroxide hydrate).

Preferably, with the method according to the invention, it is sought toobtain an hydrated uranium peroxide for which the impurity contents arecompatible with its transformation into UF₆ and for which the impuritycontents meet the ASTM C-787 standard.

The U₃O₈ powder may contain one or more of the following impurities, forexample in the following contents, expressed in ppm/U:

As: 102

Ca: 1383

Si: 2312

Zr: 316

SO₄: 29205

Mo: 1109

Na: 20

The UO₃ powder may contain one or more of the following impurities, forexample in the following contents expressed in ppm/U:

Na: 404

Ca: 407

Mo: 9

V: 5

W: 2

Cr: 30

The reactor used for applying the method according to the invention andnotably for carrying out steps a) and b) is generally a perfectlystirred reactor generally provided with a propeller stirrer, for examplea three-bladed propeller.

The reactor may further be provided with counter-blades or baffles.

The volume of the reactor may be easily selected by the man skilled inthe art according to the suspension volume which is desirably prepared.

The reactor may further be provided with sensors and devices formeasuring the values of parameters such as the pH and the temperature ofthe suspension.

The suspension is generally prepared by introducing a known amount ofpowder of oxide(s) into the reactor.

The intended amount of demineralized water is then added to this knownamount of oxide, in order to obtain a suspension having the desiredconcentration.

It is quite obvious that it is also possible to begin by introducing thedemineralized water into the reactor, and then adding the oxide powderto the demineralized water.

The concentration of oxide(s) of the suspension is generally from 10 to500 gU/L, preferably from 100 to 200 gU/L.

The pH of the demineralized water is adjusted to a value from 2 to 3 byadding an acid or a mixture of acids.

This(these) acid(s) may be any mineral or organic acid.

As this will be seen later on, an acid for which the anion further has acomplexing action which improves kinetics of the reaction, is preferred.

The preferred acids are sulfuric acid, oxalic acid and mixtures thereof.

Other acids may moreover be used for adjusting the pH but sulfuric acidhas the advantage of not introducing bothersome elements towards thenuclear purity of UF₆ since no ASTM specifications dealing with sulfurexist.

Moreover, the conversion rate into hydrated UO₄ is limited by theformation of a reaction intermediate (uranyl ion UO₂ ²⁺) but may beaccelerated by using at least one complexing anion like the sulfateanion or the oxalate anion or further the citrate anion, and/or, as thiswill be seen later on, by applying ultrasonic waves.

It is therefore possible to add a compound providing, this complexinganion during step a) and/or step b) of the method according to theinvention. In the case of sulfur, the optimum ratio S/U is 0.125.

Sulfuric acid will preferably be used as a compound providing thecomplexing anion for suspending the uranium oxide in order to obtainfast conversion kinetics.

The powder and the demineralized water having been introduced into thereactor, stirring is started in order to suspend the powder(s) in thedemineralized water.

The stirring speed is adjusted so as to allow effective suspension ofthe powder.

Stirring is continued during the whole duration of the conversion intohydrated UO₄ in order to complete crystallization of the initialuranium.

It is then possible to begin with adding oxygenated water into thesuspension.

The addition of oxygenated water may be accomplished by means of anyadequate device giving the possibility of controlling the flow rate ofoxygenated water introduced into the reactor.

The addition of oxygenated water is also preferably achieved withstirring.

Hydrogen peroxide is generally added in the form of an aqueous solutionat a concentration of 30% to 70% by weight.

The total amount of added oxygenated water is, according to theinvention, such that the stoichiometric excess of H₂O₂ relatively to theinitial uranium is from more than 1 to 10, preferably from 1.5 to 3,relatively to the stoichiometry of the following reaction (1), and frommore than 1.33 to 10 relatively to the stoichiometry of the followingreaction (2):

UO₃+H₂O₂ +nH₂O→UO₄ .nH₂O+H₂O  (1)

UO_(2.67)+1.33H₂O₂ +nH₂O→UO₄ .nH₂O+H₂O  (2)

The reaction between the oxides and the oxygenated water is exothermicand an increase in the temperature of the bath for example by about 10°C. is observed.

In an embodiment, the following successive steps b1) and b2) are carriedout:

b1) adding, as described above, hydrogen peroxide H₂O₂ to the aqueoussuspension of a UO₃ and/or U₃O₈ powder, preferably with stirring, andthen stopping the addition;

b2) ripening the suspension, preferably with stirring.

During step b1) it may be estimated that some conversion into hydrateduranium peroxide occurs, but this conversion is not total.

Step b1) may be described as a nucleation, crystallization, formation ofuranium peroxide hydrate crystallites, step.

During step b2), the conversion is continued until the conversion of UO₃and/or U₃O₈ into UO₄ hydrate is total or substantially total, forexample of more than 90% or even 99.9%.

Step b2) may be described as a step for ripening, growing thecrystallites obtained during step b1).

The duration of said step b1) may be from 1 to 8 hours, preferably from1 to 3 hours, and the duration of the step b2) may be from 1 to 24hours, preferably from 1 to 3 hours.

The total duration of steps b1) and b2) is such that the conversion intohydrated uranium peroxide is total or substantially total.

In another embodiment, the ripening step is not carried out at the endof step b1) and step b2) is omitted.

It should be noted that during the reaction of oxygenated water with theoxides, the pH varies but remains globally stable, constant, at thevalue to which it had been adjusted before adding the oxygenated waterby adding an acid, which means that it is generally not necessary to addfurther acid during step b) in order to control the pH to the intendedvalue.

In fact, it may be estimated that some regulation of the pH is inducedby the addition of H₂O₂ to UO₃ and to U₃O₈.

At the end of the reaction, the conversion being total or substantiallytotal, the pH is generally stabilized at a value for example from 1.6 to2.

At the end of step b), the conversion into hydrated uranium peroxidebeing total or substantially total, the precipitate, the crystals of UO₄hydrate, generally as UO₄ tetrahydrate, UO₄.4H₂O, or optionally as UO₄dihydrate, UO₄.2H₂O, are recovered, notably in the case when thesuspension was subject to the action of ultrasonic waves.

In a first alternative of this recovery step c) it is possible torecover, collect, the UO₄ hydrate precipitate, crystals, by removing thewater from the suspension by evaporating the latter, and the UO₄ hydrateprecipitate, crystals, are then recovered as a dry solid, generally withan humidity of less than 7% by mass, generally directly in the reactorwithout it being necessary to apply a liquid/solid separation operationsuch as a filtration. The recovered UO₄ hydrate crystals in thisalternative are generally UO₄.2H₂O crystals.

This first alternative is in particular applied, as described in detailbelow, in the case when the suspension is subject to the action ofultrasonic waves.

In a second alternative of this recovery step c), it is possible torecover, collect, the precipitate, the hydrated UO₄ crystals, byseparating them from the suspension with a liquid-solid separationoperation, in the form of a humid solid, for example with a humidityfrom 30% to 80% by mass, also called a cake.

The recovered UO₄ hydrate crystals in this second alternative aregenerally crystals of UO₄.4H₂O.

This liquid-solid separation operation may be an operation for filteringthe suspension.

This filtration operation may be achieved in vacuo or by action of acentrifugal force.

The collected humid solid may then be washed with a washing liquid.

Said washing liquid may be demineralized water or an aqueous solution,acidified, preferably at a pH from 2 to 3, for example with sulfuricacid.

An aqueous solution preferably with a pH from 2 to 3, of a complexinganion towards the impurities contained in the humid solid such as thosealready mentioned above, may also be used as a washing liquid.

Sulfuric acid has the advantage of playing both an acidifying andcomplexing role by means of its sulfate anions.

The washing operation may be repeated from 1 to 10 times depending onthe desired impurity content of the uranium peroxide.

Advantageously, the washing ratio defined by the ratio of the mass ofthe washing liquid (on the totality of the washings) to the mass of thehumid solid is from 1 to 30, preferably from 1 to 10, in order to limitthe volumes of water required for washing.

The suspension may further be subject to the action of ultrasonic waves.

The applied ultrasonic waves may have a single frequency, but acombination of ultrasonic waves with different frequencies may be used,for example a combination of high frequency ultrasonic waves, with afrequency for example of 2.4 MHz, and of low frequency ultrasonic waveswith a frequency for example of 35 kHz.

For example, it is possible to place the reactor containing thesuspension into an ultrasonic tank or else position one or severalultrasonic probes in the reactor.

Generally the suspension is subject to the action of ultrasonic waveswhile adding oxygenated water to the suspension. But it is also possibleto apply ultrasonic waves during the step for preparing the solution.

The conversion is then much faster than in the case when the reaction iscarried out without subjecting the suspension to the action ofultrasonic waves, and the duration of step b) is then only from 1 to 2hours, instead of for example 24 hours, in order to obtain total orsubstantially total conversion.

The action of the ultrasonic waves should not be mistaken for mechanicalstirring.

Generally, when ultrasonic waves are used, the suspension is not stirredin another way.

It should be noted that the action of ultrasonic waves causes anincrease in the temperature of suspension, which causes overconsumptionof H₂O₂.

When ultrasonic waves are used, it may therefore be necessary to coolthe suspension in order to avoid degradation of the oxygenated waterwhich generally occurs at a temperature above 50° C.

Because of the heating of the suspension caused by the ultrasonic waves,the water of the suspension may be removed by evaporation, and theprecipitate (the hydrated UO₄ crystals) is then recovered directly inthe reactor as a quasi-dry solid, for example with a humidity of lessthan 7% by mass and without any washing.

The use of ultrasonic waves therefore gives the possibility ofsignificantly accelerating conversion kinetics and notably reduces theamount of water in the obtained hydrated uranium peroxide. Consequently,the hydrated uranium peroxide may be recovered without it beingnecessary to pass through a liquid-solid separation step, such as a stepfor filtering the suspension.

As this separation step is suppressed, the method is thereforesimplified and shortened.

The method according to the invention may optionally comprise a step fordrying the recovered UO₄ hydrate crystals.

This drying step is generally carried out at a temperature from 60° C.to 100° C. for a period of 1 to 24 hours.

During this step, the recovered hydrated uranium peroxide is transformedinto UO₄.2H₂O if this is UO₄.4H₂O.

The obtained hydrated uranium peroxide has high reactivity for fastconversion into UF₄.

For example, a conversion of at least 90% of the uranium into UF₄ isachieved in 800 seconds. Indeed, the method according to the inventiongives the possibility of obtaining a hydrated UO₄ having a high specificsurface area which may range up to 30 m²/g.

It should be noted that the conversion rate has an influence on themorphology of the uranium peroxide hydrate obtained by the methodaccording to the invention, which generally appears in the form ofnanometric needles with a length from 300 to 500 nm and a diameter from50 to 100 nm.

Indeed, the slower the conversion, and the longer and the finer are theneedles and the more the uranium peroxide or the uranium peroxidehydrate have a high specific surface area.

The needles prepared by the method according to the invention haveacicularity expressed by the length/diameter ratio, generally from 3 to10.

The contents of impurities in the uranium peroxide obtained by themethod according to the invention, notably because the method accordingto the invention does not comprise any preliminary dissolution step thatmay bring additional impurities, are very low.

In the following Table 1 are given the initial contents of impurities inthe oxide and the final contents of impurities in the purified uraniumperoxide obtained at the end of the method according to the invention.

TABLE 1 Initial Decontamination content in Final content infactor(initial the oxide the purified UO₄ content/final Impurity (ppm/U)hydrate(ppm/U) content) Mo 1394-1414 39-40 35.0 W 141-171 19-22  8.0 S4669-4715  250-1400 3-18.8

The contents of impurities in the final peroxide are less than those ofperoxides obtained with the methods of the prior art and for most ofthem are compliant with the ASTM C-787 standard.

EXAMPLES

The following examples describe the results obtained by applying themethod according to the invention on several types of uraniferousconcentrates for which the mining origin, the chemical composition andthe calcination temperature are different.

These compounds will therefore be noted in the continuation of the textin the form of

concentrate 1

,

concentrate 2

etc.

In the examples 1 to 5 which follow, focus is laid on the conversion ofU₃O₈.

Example 1 Tests on the Concentrate 1

In this example, precipitation of uranium peroxide with 30% hydrogenperoxide is accomplished from Concentrate 1.

The targeted concentration in the reactor is 100 g/L.

The initial content of sulfates in the mined oxide is 24,824 ppm/U.

The reactor used for this precipitation is a perfectly stirred reactor

MSU 700

with a useful volume of 700 mL provided with 4 counter-blades and athree-bladed propeller stirrer, the diameter of these blades being 50mm.

The speed of rotation of the three-bladed propeller is adjusted to 600rpm in order to allow efficient suspension of the uranium powder.Precipitation of the uranium is carried out at room temperature.

The characteristics of the tank of the reactor as well as of thestirring device are indicated in Table 2 below:

TABLE 2 Tank Taps on the lid 4 Tank volume 0.7 L Tank inner diameter 80mm Tank height 135 mm Counter-blades inside the yes (4) tank Volume ofliquid in the 0.5 L tank (when operating) Stirring Stirring type Rodwith 3- fin propellers Blade height 15 mm Blade diameter 50 mm Bladeheight relatively 0.05 mm to the bottom of the tank Blade diameter/tank0.6 diameter ratio Speed of rotation of the 600 rpm three-bladedpropeller (turbulent flow)

Various sensors and measurement devices notably for the pH and for thetemperature give the possibility of tracking the precipitation reaction.

After adding a known amount of the milled oxide powder into the reactor,the uranium is suspended by stirring in demineralized water, the pH ofwhich is adjusted to pH 3 with sulfuric acid.

The oxygenated water supply is then started by means of a meteringsyringe pump allowing control of the flow rate of reagent introducedinto the reactor.

The reaction is exothermic as shown by a 10° C. increase in thetemperature of the bath, and the pH is stabilized at 1.6 at the end ofthe reaction.

After having achieved introduction of oxygenated water into the reactorfor a duration of 3 h 30 mins corresponding to a molar ratio H₂O₂/U=3,the oxygenated water supply is stopped and the obtained homogeneoussuspension of yellow hydrated UO₄ is left with stirring for ripeningduring 3 h 30 mins.

After stopping the stirring, the uranium suspension is filtered on aBüchner (filter: Ø=142 mm; porosity=0.45 μm) and then washed with wateracidified to pH 3 with sulfuric acid. The washing ratio or

wash ratio

is 1.6.

After filtering the uranium suspension, a humid cake is obtained. Thehumidity content of this cake is 63%.

Analysis of the chemical composition of the filtration mother liquorsshows that the residual uranium content in the filtrate is very low,i.e. of the order of 1 mg/L.

The obtained cake is then dried in the oven at 90° C. for 24 hours andthe dry residue is analyzed.

Analyses of the obtained solid were carried out by X-ray diffraction(XRD) (see FIG. 1) and by scanning electron microscopy (SEM) (see FIG.4).

The XRD analyses (FIG. 1) show that the dry residue actually consists ofUO₄ hydrate in the dihydrate form UO₄.2H₂O (recognition of thecharacteristic peaks of the defined compound).

The SEM photographs (FIG. 4) show that UO₄.2H₂O is in the form ofnanometric needles with a length of 200 nm for example.

The impurity contents measured in the purified UO₄ hydrates are given inthe following Table 3:

TABLE 3 Purified UO₄ Decontamination Concentrate 1 hydrate factor(ppm/U) (ppm/U) [ ]init/[ ]final As 87 10 8.7 Ca 1176 152 7.7 Si 1965330 6.0 Zr 269 37 7.3 SO₄ 24824 <7000 >3.5

Considering the whole of these results, it may be considered that theconversion of the Concentrate 1 into UO₄ hydrate is total and that theprecipitation yield is close to 100%.

The concentrations of impurities in the final product show that themethod according to the invention has allowed significant purificationof the initial concentrate.

In other words, the method according to the invention gave thepossibility of removing the essential part of the chemical elementspresent as impurities in the initial concentrate.

Example 2 Tests on the Concentrate 2

In this example, precipitation of uranium peroxide is carried out underthe same conditions as in Example 1 but on the Concentrate 2.

The same behavior of the reaction medium is observed as in Example 1,i.e.: exothermic reaction, stabilization of the pH towards 1.6 at theend of the reaction but the kinetics is much slower. The duration of themethod until the stopping of the stirring which was 7 hours (3 h30mins+3 h 30 mins) in Example 1, is 24 hours in Example 2.

The humidity content of the UO₄ hydrate cake is higher than inExample 1. This humidity content is actually 78% instead of 63%. Thisdifference is perhaps related to the size of the UO₄ hydrate needleswhich are much larger than in the case of the Concentrate 1 (see FIG.5).

The uranium content of the filtrate is 9 mg/L.

Analyses of the obtained solid were carried out by X-ray diffraction(XRD) (see FIG. 2) and by scanning electron microscopy (SEM) (see FIG.5).

In FIG. 5, 1 to 2 μm needles of UO₄.2H₂O are observed.

The measured impurity contents in the purified UO₄ hydrate are given inthe following Table 4:

TABLE 4 Purified UO₄ Decontaminating hydrate factor U₃O₈ (ppm/U) (ppm/U)[ ]init/[ ]final Mo 943 103 9.2 Na 17 4 4.3

The molybdenum content is still high as compared with the ASTMspecification and it does not seem possible to reduce it even in thecase when complexing agents are used which promote local dissolutionkinetics of the uranium (see Examples 3 and 4).

However, it is no doubt possible to improve the removal of contaminationby Mo by intensive washings of the UO₄ hydrate cake.

Washing with a washing ratio

wash ratio

of 10 may give the possibility of attaining a Mo content close to 10ppm/U, which might be acceptable in the case of a supplementarypurification downstream from the method (absorption of the impurities inUF₆).

Considering the whole of these results, it may be considered that theconversion of the Concentrate 2 is as satisfactory as the one of theConcentrate 1 but it is slower and requires an intensive washing stepfor obtaining sufficient decontamination.

Example 3 Tests on the Concentrate 3 with the Addition of Sulfates

The conversion of the Concentrate 3 was tested according to theoperating procedure of Example 1 but the experimental results show thatin this case, conversion is not possible.

The operating procedure was therefore modified by adding sulfates to theconcentrate in order to complex the uranium and allow its conversioninto UO₄ hydrate.

The tested molar ratios are: S/U=[0.125−1].

These tests were conducted on smaller amounts of U, i.e. a few grams ina stirred beaker with small volumes of solution (10 mL).

The targeted concentration in the beaker is 250 gU/L.

30% oxygenated water is gradually added to the powder suspendedbeforehand in demineralized water, with a molar ratio H₂O₂/U=2.

The sulfates are added in the form of sulfuric acid, so that the molarratio S/U is equal to 0.125 which is the optimum value of this ratio,which corresponds to a concentration of sulfates in solution of 13 g/L.

After 8 hours, the initial concentrate is completely converted into UO₄hydrate.

The pH varies during the reaction but is globally stable and equal to 2.

The formed hydrated UO₄ is filtered on a filter paper by gravity but isnot subsequently washed.

The uranium content in the filtrate is 28 mg/L.

The analysis of the purified uranium given in Table 5 below shows thatcertain impurities such as Mo or W are removed, decontaminated but onthe other hand sulfur is not removed, purified.

TABLE 5 Purified Decontamination U₃O₈ UO₄ factor (ppm/U) (ppm/U) []init/[ ]final Mo 1017 82 12.4 W 56 26 2.2 S 761 722 1.05

The results are therefore satisfactory, in particular as regards thekinetics, which is closer to that observed in the case of Example 1.

But like in the case of Example 2, intensive washing seems to benecessary in order to perfect removal of contamination of the uranium byimpurities.

Example 4 Tests on the Concentrate 4 with Addition of Oxalates

As in the case of the preceding example, tests were conducted by usingoxalic acid for accelerating conversion of the Concentrate 4.

The tested molar ratios are C₂O₄/U=[0.05−1].

The targeted concentration in the beaker is 50 gU/L.

The 30% oxygenated water is gradually added to the powder suspendedbeforehand in demineralized water, with a molar ratio H₂O₂/U=2.

Oxalic acid is added to the medium so that the molar ratio C₂O₄/U isequal to 0.025, which corresponds to a concentration of oxalates insolution of 2.6 g/L.

After 11 hours, the initial U₃O₈ is completely converted to hydratedUO₄.

The pH does not fall below 2.

The hydrated UO₄ formed is filtered on a paper filter by gravity but isnot washed subsequently.

The uranium content in the filtrate is very high, i.e. 520 mg/L, and maybe explained by the highly complexing nature of the oxalate ions.

Additional tests were therefore conducted with an oxalate/sulfatemixture in order to attempt to reduce the uranium release, since thesulfates are less complexing than oxalates.

In the case of a ⅓ oxalic acid-⅔ sulfuric acid mixture, the conversionrate is 95% after 8 hours 30 mins.

The releases are reduced to 330 mg/L and therefore remain approximatelyten times higher as compared with tests conducted without addition ofoxalic acid.

The analysis of the purified uranium (see XRD spectrum: FIG. 3) when a ⅓oxalic acid-⅔ sulfuric acid mixture is used, is given in Table 6 below:

TABLE 6 ⅓ oxalic Purified acid-⅔ UO₄ Decontamination sulfuric acid U₃O₈hydrate factor mixture (ppm/U) (ppm/U) [ ]init/[ ]final Mo 1011 132 7.7W 18 6 3 S 406 562 — V 42 44 — Zr 1019 953 1.1

The decontamination factors observed for W and Mo are comparable withthose observed in the tests conducted with sulfuric acid alone.

Moreover, this treatment does not give the possibility ofdecontaminating, removing the following elements: S, V, Zr.

The operating procedure applied in this example, therefore seems to beless adapted than the one used in Example 3 in the case of highly impureoxides.

Example 5 Tests on the Concentrate 2 with Ultrasonic Waves

Tests were carried out by placing the beaker in an ultrasonic bath (35kHz) in order to accelerate conversion of the Concentrate 2 according tothe operating procedure of Example 3.

The conversion into UO₄ hydrate is carried out much more rapidly than inExample 2, i.e. within 2 hours instead of 24 hours, but the observedincrease in temperature induces an overconsumption of H₂O₂, whichexcludes any optimization of this molar ratio (H₂O₂/U) with the penaltyof reducing the conversion rate.

Moreover, it is seen that the final product is practically dry (nofiltrate).

By using ultrasonic waves, it is therefore possible to significantlyaccelerate the conversion kinetics and to reduce notably the amounts ofwater in the purified UO₄ hydrate, which may have an advantage bysimplifying the method, with potential suppression of a filtration step.

This operating procedure may be optimized (preliminary milling,injection of an inert gas, high/low frequencies ultrasonic wavescombinations, control of the temperature by cooling the reaction medium,increase in the L/S ratio).

In the following Example 6, focus is laid on the conversion of UO₃.

Example 6 Tests on the Concentrate 5

Uranium peroxide precipitation tests were conducted under the sameconditions as in Example 1 but with the Concentrate 5 by addingadditional calcination and washing steps according to the followingscheme:

The step for calcination of impure UO₄ hydrate into UO₃ at 200° C. for 3hours allows modification of the structure of the uranium in order to beable to then purify it more easily from its impurities during thesubsequent steps.

The temperature of the calcination is a sensitive parameter.

Indeed, a test conducted at a temperature of less than 200° C. (150° C.)demonstrated degradation of the results obtained by the method accordingto the invention as regards the purification level of the uraniumtowards certain impurities such as sodium (imperfect calcination ofimpure UO₄ hydrate into UO₃, residual fraction of impure UO₄ hydratewhich is more difficult to decontaminate as regards sodium removal).

The second step consists of recrystallizing UO₃ into UO₄ hydrateaccording to the initial operating procedure of Example 1 and then ofwashing the obtained cake with higher washing ratios (

Wash Ratios

or

WR

) than previously.

Thus, a maximum wash ratio WR of 25 was applied but an optimum washratio may be located between 1 and 10.

The washing is carried out with demineralized water for which the pH isadjusted to 2.5 with sulfuric acid.

The selection of the acidity of the pH is guided by the efficiency ofthe purification in the washing step.

Indeed, a decrease in the purification level of the uranium with regardto sodium is observed if the washing is carried out with distilled waterwithout any sulfuric acid.

This is explained by the complexing nature of the sulfates towardsimpurities such as sodium.

The washings are carried out with the method of successive “repulping”operations, with a repulping duration of 600 seconds, and the UO₄hydrate suspensions are filtered between two repulping operations on aBüchner (filter: Ø=142 mm; porosity=0.45 μm).

With this method, the precipitation kinetics are fast, i.e. less than 5hours (an estimation of the minimum conversion time is located around 1hour), and the analyses of the obtained solid show that the conversionis actually total.

The reaction is exothermic and the pH increases at the beginning of thereaction and then returns to its initial value, which confirms theassumptions of precipitation of hydrated UO₄ catalyzed by the acidaccording to the following reaction scheme:

Formation of the uranyl ion UO₃ + 2H⁺→ UO₂ ²⁺ + H₂O (Reactionintermediate): Precipitation of the UO₄ UO₂ ²⁺ + nH₂O + H₂O₂→ UO₄•nH₂O +2H⁺ hydrate: Equation balance: UO₃ + nH₂O + H₂O₂ → UO₄•nH₂O + H₂O

The targeted concentration in the reactor was 100 gU/L in this example,but other tests have shown that the optimum concentration should befound in the interval 100-200 gU/L.

Actually, a regression of the purification level of sodium is observedif the targeted [U] is greater than 200 gU/L.

This regression is the effect of the decrease in the specific surfacearea of the UO₄ hydrate with the concentration.

The oxygenated water excess used in this example is the same as inExample 1 but the optimum molar ratio H₂O₂/U is located between 1 and 3.

The humidity level of the hydrated UO₄ cake is 46%.

The morphology and the size of the hydrated UO₄ grains are differentfrom the others hydrated UO₄ produced in the previous examples: thepowder consists of small nanometric agglomerates, for example from 100nm to 200 nm (see FIG. 6).

The uranium content in the filtrate is 5.5 mg/L.

As shown in the following Table 7, the purity of the obtained UO₄hydrate is highly satisfactory with respect to the constraints on themethod related to the ASTM standards, and to the Comurhex® fluorinationmethod (CX method).

TABLE 7 Purified Impure hydrated hydrated UO₄ UO₄ ASTM CX method (ppm/U)(ppm/U) standard limit Na 404 4 40 Ca 407 58 Mo 9 1 1.4 V 5 4 1.4 W 2 11.4 Cr 30 3.9 10

1. A method for converting UO₃ and/or U₃O₈ into hydrated UO₄ of formulaUO₄.nH₂O wherein n is 2 or 4, comprising the following successive steps:a) preparing an aqueous suspension of a UO₃ powder and/or a U₃O₈ powder;b) adding hydrogen peroxide H₂O₂ to the aqueous suspension of a UO₃and/or U₃O₈ powder, converting the UO₃ and/or U₃O₈ into hydrated UO₄ andprecipitating, crystallizing the hydrated UO₄ in the suspension; c)recovering the precipitate, crystals of UO₄ hydrate; d) optionally,washing the recovered UO₄ hydrate precipitate, crystal(s); e)optionally, repeating step d); f) optionally, drying the precipitate,the crystals; wherein the addition of H₂O₂ to the aqueous suspension iscarried out so that the suspension contains a stoichiometric excess ofH₂O₂ relatively to the stoichiometry of the reaction from UO₃:UO₃ +nH₂O+H₂O₂→UO₄ .nH₂O+H₂O  (1) or of the reaction from U₃O₈UO_(2.67)+1.33H₂O₂ +nH₂O→UO₄ .nH₂O+H₂O  (2), and the pH of thesuspension is maintained in steps a) and b) at a value comprised between2 and
 3. 2. The method according to claim 1, wherein the pH of thesuspension is adjusted during step a) to a value comprised between 2 and3 by adding an acid to the suspension.
 3. The method according to claim1, wherein the acid is selected from oxalic acid, sulfuric acid andmixtures thereof.
 4. The method according to claim 1, wherein thestoichiometric excess of H₂O₂ is from more than 1 to 10, relatively tothe stoichiometry of the reaction (1) and from more than 1.33 to 10relatively to the stoichiometry of reaction (2).
 5. The method accordingto claim 1, wherein the hydrogen peroxide is added as an aqueoussolution at a concentration of 30% to 70% by weight.
 6. The methodaccording to claim 1, wherein the aqueous suspension of UO₃ and/or U₃O₈has a uranium concentration from 10 to 500 g/L (gU/L).
 7. The methodaccording to claim 1, wherein steps a) and b) are carried out withstirring.
 8. The method according to claim 1, wherein during step a)and/or step b) complexing anions are added to the suspension.
 9. Themethod according to claim 8, wherein the complexing anions are selectedfrom sulfate anions, oxalate anions and mixtures thereof.
 10. The methodaccording to claim 1, wherein the duration of step b) is selected sothat the conversion of UO₃ and/or U₃O₈ into UO₄ hydrate is total orsubstantially total.
 11. The method according to claim 1, wherein stepb) comprises the following successive steps b1) and b2): b1) addinghydrogen peroxide H₂O₂ to the aqueous suspension of a UO₃ and/or U₃O₈powder and then stopping the addition; and b2) ripening the suspension.12. The method according to claim 11, wherein the duration of step b1)is from 1 to 8 hours and the duration of step b2) is from 1 to 24 hours.13. The method according to claim 1, wherein the addition of hydrogenperoxide H₂O₂ is carried out during the whole duration of step b). 14.The method according to claim 13, wherein the duration of step b) isfrom 1 to 8 hours.
 15. The method according to claim 1, wherein duringsteps a) and/or b) the suspension is subject to the action of ultrasonicwaves.
 16. The method according to claim 1, wherein the water of thesuspension is removed by evaporation and the precipitate, the crystalsof UO₄ hydrate, are recovered as a dry solid, with a humidity of lessthan 7% by mass, generally consisting of UO₄.2H₂O.
 17. The methodaccording to claim 1, wherein during step c), the precipitate, thecrystals of UO₄ hydrate, are separated from the suspension by asolid/liquid separation operation, as a humid solid, with a humidityfrom 30 to 80% by mass, generally consisting of UO₄.4H₂O.
 18. The methodaccording to claim 17, wherein the humid solid is washed at least oncewith a washing liquid.
 19. The method according to claim 18, wherein thewashing liquid is selected from demineralized water; acidified aqueoussolutions at a pH from 2 to 3; and solutions containing an agentcomplexing the impurities contained in the humid solid.
 20. The methodaccording to claim 19, wherein the washing ratio defined by the ratio ofthe mass of the washing liquid to the mass of the humid solid is from 1to
 30. 21. The method according to claim 1, wherein the UO₃ oxide and/orthe U₃O₈ oxide appear in the form of a uraniferous concentrate called

yellow cake

, or the UO₃ oxide and/or the U₃O₈ oxide stem from drying, and then fromcalcination of a uranium concentrate based on hydrated UO₄, on ammoniumdiuranate, or on uranium tricarbonate obtained by precipitation in areactor, from an uraniferous solution.